1. Field of the Invention
This invention relates generally to investigation of subsurface earth formations, and, more particularly, to a method for correcting data obtained by a downhole tool for anomalies that may be caused by environmental characteristics and/or intrinsic downhole tool parameters. The invention has general application in the well logging art, but is particularly useful at a well site while logging.
2. Description of Related Art
A major goal of well logging is to maximize the amount of hydrocarbons recovered from an earth formation. By continuously monitoring oil saturation in the earth formation, secondary and tertiary techniques may be employed to enhance recovery of hydrocarbons. Oil saturation is usually expressed as a percentage by volume of oil in the pore space. Different methods have been developed for monitoring oil saturation during production of a well.
One method for monitoring oil saturation is based on the fact that hydrocarbons contain carbon and water contains oxygen. A carbon/oxygen ratio (xe2x80x9cCORxe2x80x9d) is used to compute oil saturation. The COR is derived by applying a spectral fitting technique to an inelastic gamma ray spectrum to compute carbon, oxygen, and other elements present in the formation. This approach provides one means for computing the COR.
Alternatively, the COR is derived using counts from broad energy regions xe2x80x9cwindowsxe2x80x9d in the inelastic gamma ray spectrum across the region of the predominant carbon and oxygen gamma ray energies. The COR is derived by taking the ratio of the counting-rates in two energy windows of the inelastic gamma ray spectrum. Such measurements will be referred to herein as xe2x80x9cwindows COR valuexe2x80x9d measurements. All gamma rays in these windows do not result solely from the elements carbon (C) and oxygen (O). Nevertheless, this count-rate windows COR value will respond to changes in oil saturation in the formation, provided the other formation and borehole properties remain constant.
The conversion between the spectrally fitted COR and the windows COR value and the oil-saturation value is typically determined by making many measurements with one downhole tool (called the xe2x80x9cdatabasexe2x80x9d or xe2x80x9ccharacterizationxe2x80x9d tool) at standard conditions in laboratory simulated formations having accurately known porosity, lithology, completion geometry, and saturation. This database set of measurements is commonly known as the xe2x80x9ctool characterization.xe2x80x9d The tool characterization may also be derived by theoretical modeling techniques as known in the art.
Gamma ray photon energies are detected downhole with the use of conventional downhole tools such as the Reservoir Saturation Tool (RST), a mark of Schlumberger (see U.S. Pat. No. 4,937,446, assigned to the present assignee), or any similar tool. To provide the COR measurements, these tools are disposed downhole to irradiate the borehole and surrounding formation with high-energy neutrons. Detectors in conventional downhole tools typically use scintillation crystals, such as thallium-activated sodium iodide (NaI), thallium-activated or sodium-activated cesium iodide, bismuth germanate (BGO), gadolinium oxyorthosilicate doped with cerium (GSO), and the like. The gamma ray detector or detectors in the tool measure gamma ray photons produced from carbon (C) and oxygen (O) during the neutron burst as a result of neutron inelastic scattering from the nuclei of carbon (C) and oxygen (O) present in the formation and the borehole. Analyzing the inelastically produced gamma ray photon energy spectrum for the characteristic energy gamma ray photons from atomic elements such as carbon (C), oxygen (O), silicon (Si), calcium (Ca), iron (Fe), and the like, allows the presence of these elements, and their relative abundance, in the formation and borehole regions, to be quantified.
When the formation water salinity is known and is higher than about 20000 parts per million (20 kppm) sodium chloride (NaCl), a different pulsed-neutron technique may be used to measure the rate of capture of thermal neutrons. This quantity, known as the thermal-neutron capture cross-section (Sigma or xcexa3), is strongly influenced by the affinity of chlorine (Cl), a primary constituent of saltwater, to absorb thermal neutrons. Conventional pulsed-neutron tools measure xcexa3 by measuring the counting rate of gamma ray photons produced by thermal neutron capture after a pulse of neutrons has been released into the formation.
Typical uses of COR measurements are to detect bypassed reserves and monitor them with repeated measurements during the life of the reservoir (time-lapse monitoring). These reservoirs typically have older wells having fresh or unknown salinity environments. For this application, the inherent tool-to-tool accuracy found with conventional downhole tools is typically +/xe2x88x9210 s.u. (expressed in saturation units, s.u., as a percentage by volume of oil in the pore space) in reservoir-like rocks. For the typical applications mentioned above, this accuracy is usually adequate. However, some applications of COR logging such as time-lapse monitoring in very large fields, which may include steam injection, require improved accuracy. These large monitoring projects may require logging 400-500 observation wells every year. Clearly, this necessitates using multiple downhole tools. Also it cannot be guaranteed that the same tool will necessarily log the same well, year after year. Since the COR measurements are used in time-lapse (differential) mode, a technique is needed to calibrate or normalize the readings of the various downhole tools to the database tool. In other words, a method is needed to remove or reduce the +/xe2x88x9210 s.u. tool-to-tool accuracy variations mentioned above.
Typically, there exist some real, quantifiable spectral differences between the downhole tools and the database tool, for otherwise, all tools would read the same and the typical +/xe2x88x9210 s.u. differences would not be observed. This variability in tool-to-tool accuracy may result from buildup of mechanical tolerances, as well as slightly differing properties of scintillation crystals, photomultiplier tubes, shielding, and fast detector electronics and associated regulation loops.
One conventional field technique used in the industry to help improve the accuracy of these measurements involves taking a COR reading in a xe2x80x9cknown water sandxe2x80x9d in a given reservoir. Since the presumed correct COR reading here is zero, any non-zero reading is noted and this single value, the xe2x80x9cCOR offset,xe2x80x9d is globally subtracted from the measured COR reading at every depth-level logged. This conventional technique assumes that the COR offset is the same for all formation and borehole conditions - an assumption which, more likely than not, may not be true.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the invention, a method of calibrating readings of a downhole tool during well-logging is provided, the method including collecting data representative of gamma ray photon energies detected by a downhole tool during well-logging, a spectrum of the gamma ray photon energies detected by the downhole tool having a derivative with respect to the gamma ray photon energies. The method also comprises calibrating the readings of the downhole tool using at least one predetermined correction factor based on the derivative of the spectrum of the gamma ray photon energies detected by the downhole tool.
In another aspect of the invention, a computer-readable, program storage device is provided, encoded with instructions that, when executed by a computer, perform a method of calibrating readings of a downhole tool, the method including collecting data representative of gamma ray photon energies detected by a downhole tool during well-logging, a spectrum of the gamma ray photon energies detected by the downhole tool having a derivative with respect to the gamma ray photon energies. The method also comprises calibrating the readings of the downhole tool using at least one predetermined correction factor based on the derivative of the spectrum of the gamma ray photon energies detected by the downhole tool.
In yet another aspect of the present invention, a computer programmed to perform a method of calibrating readings of a downhole tool is provided, the method including collecting data representative of gamma ray photon energies detected by a downhole tool during well-logging, a spectrum of the gamma ray photon energies detected by the downhole tool having a derivative with respect to the gamma ray photon energies. The method also comprises calibrating the readings of the downhole tool using at least one predetermined correction factor based on the derivative of the spectrum of the gamma ray photon energies detected by the downhole tool.